The present invention relates to a scanning optical system which is employed for a beam scanning unit of a device such as a laser printer, and in particular, to a reflective scanning optical system which dynamically deflects (scans) a beam in a main scanning direction by a rotating polygon mirror and focuses the scanned beam on a scan target surface (photosensitive drum, etc.) via a single fθ optical element having a first surface as a refracting surface and a second surface as a reflecting surface.
A reflective scanning optical system of this type has been disclosed in U.S. Pat. No. 5,963,355, for example. FIG. 11 is a perspective view showing the overall configuration of the reflective scanning optical system disclosed in the publication. A laser beam emitted by a laser diode (semiconductor laser) 10 as a diverging beam is collimated by a collimator lens 20 into a collimated beam and then converged by an anamorphic lens 30 in an auxiliary scanning direction only.
The laser beam emerging from the anamorphic lens 30 is reflected by a flat mirror 31 toward a rotating polygon mirror 50. The laser beam incident on a reflecting surface of the rotating polygon mirror 50 is reflected by the reflecting surface and thereby dynamically deflected in the main scanning direction. The laser beam dynamically deflected by the polygon mirror 50 is then focused on a photosensitive drum 60 by a single fθ optical element 40. The fθ optical element 40 has a first surface 41 for refracting the laser beam and a second surface 42 for reflecting the laser beam.
However, since the polygon mirror 50 is manufactured by cutting and grinding each of its reflecting surfaces, variations tend to occur in the distance from the rotation axis to each reflecting surface 51. In the reflective scanning optical system described above, the variations in the distance from the rotation axis of the polygon mirror 50 to each reflecting surface 51 causes variations in the position (displacement) of a deflecting point (where the laser beam is deflected) on each reflecting surface 51 of the polygon mirror 50 in the auxiliary scanning direction, by which the interval between the scan lines on the scan target surface (photosensitive drum 60) becomes uneven and thereby the print quality is deteriorated (i.e., jitters in the auxiliary scanning direction are caused).
FIG. 12 shows a configuration of the reflective scanning optical system of FIG. 11 with regard to the auxiliary scanning direction. As shown in FIG. 12, when a reflecting surface of the polygon mirror 50 is at the position shown with the solid line 51, the beam reflected by the reflecting surface 51 reaches a point A on the photosensitive drum 60 via the path shown with the solid lines. However, if the reflecting surface shifts to the position shown with the broken line 51A, the beam reflected by the reflecting surface 51A reaches a point B on the photosensitive drum 60 via the path shown with the broken lines.
Therefore, such variations causes uneven scan line intervals and deteriorated print quality. Since the displacement of the deflecting point in the auxiliary scanning direction translates into the deterioration on the scan target surface (uneven scan line intervals, etc.) depending on the magnification of the fθ optical element 40, the deterioration becomes more serious as the magnification of the fθ optical element 40 gets larger.
Further, the second surface 42 is formed in a “twisted” shape (i.e., a sectional form in the main scanning direction changes depending on the height in the auxiliary scanning direction) in order to correct “skew distortion” which is caused by rotation of the beam around its central axis (due to the separation of the incident beam and the reflected beam in the auxiliary scanning direction).
Therefore, if the reflecting point (deflecting point) moves in the auxiliary scanning direction due to the variations in the distance from the rotation axis of the rotating polygon mirror 50 to each reflecting surface, a beam incident position on the second surface 42 measured in the auxiliary scanning direction changes depending on which reflecting surface of the polygon mirror 50 reflects the beam, by which a scan width on the scan target surface fluctuates and thereby the print quality is deteriorated (i.e., jitters in the main scanning direction are caused).